(d) Internal stress
The direct determination of stress level in a continuum such as rock, con-
crete, or soil presents the most serious difficulties. Theory dictates the use
of a disc-shaped sensing element of high aspect ratio. Material constraints
render impossible the ideal of a sensor which exactly matches, at all stress
levels, the stiffness of the continuum in which it is emplaced. The greatest
circumspection is thus required in interpreting pressures or stresses
recorded in the field. Commercially available devices for use in concrete
and rock generally consist of an oil-filled metal disc linked to a pressure
transducer which responds to fluid pressure as a function of external stress
on the face of the disc. Similar devices are sometimes employed in embank-
ment fills, an alternative being the hollow disc earth pressure cell, incorpo-
rating electrical strain gauges mounted on the internal faces of the cell.
Indirect determination of stress from measured strains, e.g. in mass
concrete, is similarly fraught with serious difficulties associated with non-
linear and time-dependent stress–strain response. Uncertainties associated
with stress determination severely degrade the value of the recovered
data, and the installation of field instrumentation for this purpose is
seldom advisable. (Instrumentation of this nature is generally associated
with research projects.)
7.2.5 Indirect investigation of leakage and seepage
It is frequently necessary to investigate and locate suspected leakage and
seepage paths within the body of an embankment or through it’s founda-
300 DAM SAFETY: INSTRUMENTATION AND SURVEILLANCE
Fig. 7.5 Hydraulic point-settlement cell
